Optical lithography at UV wavelengths is the standard process for patterning 90-nm state-of-the-art devices in the semiconductor industry, and extensions to 45-nm and below are currently being explored. Advanced lithographic schemes are focused on the use of a short UV wavelength (e.g., 193 nm or 157 nm), coupled with immersion to further reduce the effective wavelength.
Mass-produced semiconductor manufacturing entered the era of nanopatterning with UV optical lithography when the smallest feature sizes crossed the 100-nm threshold. In the last two years advanced devices have had their half-pitch at 90 nm and, according to the international roadmap for semiconductors (ITRS), this trend will continue unabated for at least one more decade with expected resolution decreasing to 65 nm in 2007, 45 nm in 2010, and 32 nm in 2013.
Until the late 1980s, the radiation sources were high-power mercury lamps, first at 436 nm, followed by 365 nm, and finally 254 nm. Then, a transition took place to the krypton fluoride excimer lasers at 248 nm, and more recently to the argon fluoride excimer lasers at 193 nm, and eventually to molecular fluorine lasers at 157 nm. Today, both 193- and 157-nm lithography is the subject of intense development, and significant progress is being made towards implementation at these wavelengths.
Several years ago a new technology was proposed, that of liquid immersion lithography, which, when implemented at 193 nm, would provide similar resolutions as “dry” 157 nm, without the risks involved in 157-nm lithography. Since early 2003, 193-nm liquid immersion lithography has taken center stage as the next generation of optical lithography. It now appears that “dry” 157-nm lithography will become a backup technology to 193-nm liquid immersion while liquid immersion 157-nm lithography is a likely candidate to be the successor to the liquid 193-nm lithography.
Submicrometer-scale optical imaging typically utilizes close proximity (<1 cm) between the focal plane and the final element of the imaging optics. Whereas normally this small space between the focal plane and the final optical element is filled with air, when it is filled with a fluid possessing a refractive index appreciably higher than 1.0, smaller features can be resolved and hence the imaging system exhibits improved resolution. This phenomenon has been well recognized for many years and this type of optical imaging is generally referred to as “immersion lithography” because it requires the focal plane to be immersed in the high-index fluid.
Liquid immersion lithography involves the introduction of a fluid between the last optical element and the photoresist surface. The effective wavelength of the imaging system is reduced in proportion to the index of refraction of the liquid. Since the performance of projection optics is essentially limited by diffraction, the shorter effective wavelength (λeff=λo/nf in a fluid of index nf) enables a higher resolution when the vacuum wavelength λo and θ, the angle of propagation between the lens and the photoresist, remain constant. The second benefit of liquid immersion lithography is the increased depth of focus, even at dimensions that can be patterned in air. For a fixed feature size, θ in the fluid is smaller than in air, and consequently the aerial image is less sensitive to displacements of the photoresist surface along the optical axis. This reduced sensitivity is equivalent to a larger depth of focus. Thus, liquid immersion lithography allows for higher resolution or increased depth of focus relative to dry lithography.
In dry lithography, the largest NA possible is 1.0, which is defined in part by the refractive index of air being 1.0. In liquid immersion lithography, a fluid is introduced between the last optical element and the photoresist and in so doing, the NA of the projection optics is effectively increased to above 1.0. For example, the 193-nm refractive index of high-purity water is ˜1.44, and this is therefore the upper limit of the NA using water at 193 nm. Viewed another way, the vacuum wavelength of 193 nm is reduced by the refractive index of water to an effective wavelength of 134 nm in the image plane. This value is less than 157 nm, implying that a higher resolution is possible with 193 nm and water immersion than with dry 157 nm. A similar reduction in effective wavelength would be possible employing liquid immersion at 157 nm.
Manufacturing of integrated circuits has been enabled by high-performance spin-on organic polymeric photoresists. The development of polyhydroxystyrene based resists was necessary to overcome high novolac absorbance at 248 nm and enable the introduction of 248 nm lithography into IC manufacturing. In a similar manner, 193-nm lithography required the development of a new polymer system to overcome the high 193-nm absorbance of phenolic-based polymers. Two different classes of polymers, polyacrylate and polycyclic copolymer based resists have been developed, and now compete for predominance in 193-nm lithography. Due to the high absorbance at 157 nm of polyhydroxystyrene, polyacrylate, and polycyclic copolymer based resists, the use of any of these resists will only be possible if the coated resist thickness is under 100 nm. This has led to the development of fluorinated polymers as resist materials capable of high resolution. Liquid immersion lithography can utilize some of the same types of photoresists as employed in dry lithography although there are concerns about leaching of chemicals from the photoresists and the effect of that leaching on resist resolution and optical lens contamination.
One difficulty associated with developing high refractive index fluids for immersion lithography relates to solving the requirement for a fluid to have both high index and low absorbance. For example, the addition of fluorine to a liquid's molecular formula will reduce not only the molecule's absorbance but also its refractive index. Current liquids employed for 157-nm immersion lithography are fluorocarbon or fluorohydrocarbon based and have refractive indices at 157-nm of less than 1.35.
Hence, there is still a need for improved immersion liquids suitable for use in immersion lithography at very short wavelengths, e.g., at 157 nm. There is also a need for such liquids that exhibit not only a high refractive index but also a relatively low absorbance.